Do you mean factory reset? Not sure. If root is built into the image it will stay for sure. Otherwise I do not know of root retains through reset. If you just mean powercycling, then yes it will retain root.
It is also easy to unroot. Typically easy one click root programs have an unroot option.
It depends on the method you use to root it. If you flash a pre-exsisting ROM that has the root already, you're always going to be rooted, unless you use something like a temporary unroot to use a service that doesn't let you when it is and maybe forget to turn it off (just my thought anyway).
If you just use an app that finds an exploit, then yes, it will be gone if you reset it, or if an update somehow gets through.
And yes, it's very easy to put a phone back to normal after you root it, as long as you have access to original images and recoveries. You'll most likely find all this at XDA. Sometimes it's just flashing the original ROMs with the recoveries attached to them, other times you have to use something like ODIN.